The present invention relates to the field of low-resistance resistors (hereafter referred to as xe2x80x9cresistorsxe2x80x9d) used for detecting current in a current-carrying circuit as a voltage, and their manufacturing method.
The conventional resistor of this type is disclosed in Japanese Laid-open Patent Publication No. H6-20802.
A conventional resistor is described below with reference to drawings.
FIG. 29(a) is a perspective, and FIG. 29(b) is a sectional view of the conventional resistor.
In FIGS. 29(a) and (b), a resistor element 1 is a rectangular parallelepiped resistance metal made of an alloy of nickel, chromium, aluminum, and copper, and it has an integrated structure with opposing ends 2 and 3. A conductive material such as solder is coated on both ends 2 and 3 of the resistor element 1, typically by plating, to form terminals 4 and 5. A central portion 6 is the central area of the resistor element 1, excluding the terminals 4 and 5, and this central portion 6 is bent against the terminals 4 and 5 in order to create a gap between the resistor and a substrate when the resistor is mounted on the substrate. An insulating material 7 is provided on the central portion 6 of the resistor element 1.
A method for manufacturing the conventional resistor configured as above is described below.
FIGS. 30(a) to 30(e) are process charts illustrating the manufacturing method of the conventional resistor. In FIG. 30(a), the rectangular parallelepiped resistor element 1 having an integrated structure made of an alloy of nickel, chromium, aluminum, and copper with a predetermined resistance is formed.
In FIG. 30(b), a conductive material 8 is plated on the entire face of the resistor element 1 (not illustrated).
In FIG. 30(c), the conductive material 8 coated on the central portion 6 of the resistor element 1 is scraped off with a wire brush so as to expose the resistor element 1 at the central portion 6.
In FIG. 30(d), the terminals 4 and 5 disposed at the sides of the resistor element 1 are bent downward against the central portion 6 of the resistor element 1.
Lastly, in FIG. 30(e), the central portion 6 of the resistor element 1 is covered with an insulating material 7 by molding to complete the conventional resistor.
The above conventional resistor achieves the integrated structure of the resistor element 1 and terminals 4 and 5 by bending the resistance metal, and the resistor element 1 is made of an alloy of nickel, chromium, aluminum, and copper. The terminals 4 and 5 are configured by plating a conductive material such as solder on the surface of both ends 2 and 3.
The electrical conductivity of the alloy of nickel, chromium, aluminum, and copper configuring the resistor element 1 has lower electrical conductivity than metals generally having good conductivity such as copper, silver, gold, and aluminum. Since the base material of the terminals 4 and 5 is made of the same alloy as that of the resistor element 1, the base material configuring the terminals 4 and 5 has a larger resistance in proportion to its smaller electrical conductivity compared to metals generally having good conductivity. Accordingly, both ends 2 and 3 of the resistor element 1 are coated, such as by plating, with a conductive material such as solder in order to reduce resistance.
In the case of resistors having large resistance in the conventional configuration, resistance at the terminals 4 and 5 is reduced by coating a conductive material such as solder on the surface of both ends 2 and 3 of the resistor element 1, and thus the difference in resistance between the resistor element 1 and terminals 4 and 5 becomes extremely large. Consequently, the composite resistance of the resistor element 1 and terminals 4 and 5, which is the overall resistance of the resistor, may be represented by only the resistance of resistor element 1, allowing to ignore the resistance at the terminals 4 and 5.
However, in the case of resistors with a resistance of 0.1 ohms or below, the resistance of the terminals 4 and 5 in the entire resistor cannot be ignored. For accurate measurement of the resistance of a resistor with a high resistance, the four-probe method is generally used. However, for measuring the resistance of a resistor with a resistance of 0.1 ohms or below, the resistance varies according to the position of the probe contacting the terminals 4 and 5, even the four-probe method is used, because the resistance of the terminals 4 and 5 affect the resistance of the entire resistor with increasing resistance of the terminals 4 and 5. In this case, fluctuation in resistance due to deviation in the measuring point on the terminals 4 and 5 increases as the proportion of the resistance of the terminals 4 and 5 in the entire resistor increases. Accordingly, it is necessary to specify the measuring point for reproducing measurements with high accuracy in the conventional configuration. However, assuring the reproducibility of the same measuring point is extremely difficult even when the measuring point is specified, thus decreasing the reproducibility of the resistance measurements.
The present invention aims to address the above disadvantage of the prior art, and provides a resistor which assures highly accurate measurement of resistance even if the measuring point is not precisely placed.
To solve the aforementioned disadvantage of the conventional resistor, the resistor of the present invention comprises a sheet metal resistor element and separate metal terminals electrically connected to both ends of the sheet resistor element. These terminals are made of metal having the same or greater electrical conductivity than that of the resistor element. The terminals and the resistor element are connected through a third metal.
With the above configuration, resistance of the terminals can be made smaller than that of the resistor element because the terminals are made of a material having the same or greater electrical conductivity than that of the resistor element. This enables to reduce the proportion of resistance of the terminals in the entire resistor, allowing to ignore its effect on fluctuation of resistance due to deviation in measuring points of a resistance measuring terminal. The present invention can thus assure reproducibility of highly accurate measurement of resistance, providing the resistor which assures highly accurate measurement of resistance even if the measuring point is not precisely placed.